tautomer

tautomer

(tô`təmər), one of two or more structural isomersisomer, in chemistry, one of two or more compounds having the same molecular formula but different structures (arrangements of atoms in the molecule). Isomerism is the occurrence of such compounds. Isomerism was first recognized by J. J. Berzelius in 1827......Click the link for more information. that exist in equilibrium and are readily converted from one isomeric form to another. Of the various types of tautomerism that are possible, two are commonly observed. In keto-enol tautomerism a simultaneous shift of electrons and a hydrogen atom occurs; it was first observed by K. Meyer in the ethyl ester of 3-oxobutanoic acid (ethyl acetoacetate), which occurs naturally as a mixture of the two forms. Ring-chain tautomerism, first recognized by Emil Fischer, is exhibited by glucose. It arises as a result of the aldehyde group (-CHO) in a sugar chain molecule reacting with one of the hydroxy groups (-OH) in the same molecule to give it a cyclic (ring-shaped) form.

In general the Schiff bases that are derived from salicylaldehyde exist mainly as the enol-tautomer with a variable contribution from the keto-tautomer (depending on the substituents) while those derived from 2- hydroxynaphthaldehyde are dominated by the keto- tautomer structure [5].

The previous studies indicate that the keto- tautomer is the more stable structure in both HNQ [7- 25] and HNQ-azo [26-31] (Fig.

3] showed the following characteristics: we assigned a complex series of signals at one to four parts per million to the various aliphatic protons, the number of peaks being doubled because both enol and keto tautomers were present in approximately equal amounts, a multiplet at 6.

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